In applicant's co-pending Published International Application WO92/22938 there is disclosed a dual polarisation waveguide probe system in which a waveguide is incorporated into a low-noise block receiver in which two probes are located for receiving linearly polarised energy of both orthogonal senses. The probes are located in the same longitudinal plane on opposite sides of a single cylindrical bar reflector which reflects one sense of polarisation and passes the orthogonal signal with minimal insertion loss and then reflects the rotated orthogonal signal. The probes are spaced .lambda./4 from the reflector. A reflection rotator is also formed at one end of the waveguide using a thin plate which is oriented at 45.degree. to the incident linear polarisation with a short circuit spaced approximately a quarter of a wavelength (.lambda./4) behind the leading edge of the plate. This plate splits the incident energy into two equal components in orthogonal planes, one component being reflected by the leading edge and the other component being reflected by the waveguide short circuit. The resultant 180.degree. phase shift between the reflected components causes a 90.degree. rotation in the plane of linear polarisation upon recombination so that the waveguide output signals are located in the same longitudinal plane.
The above waveguide probe system has been found to perform well for the purpose for which it was designed; to provide significant signal isolation better than 40 dBs. across the current Astra satellite bandwidth being 10.7-11.8 GHz. and across other bandwidths such as 11.7-12.2 GHz. for DBS and 12.2-12.75 GHz. However, there has been a trend to increase the frequency range transmitted by new satellite systems. In fact, the frequency bandwidth is planned to increase from 10.7-11.8 GHz. to 10.7-12.75 GHz. on the Astra system in the near future. With the aforementioned design it has hitherto been difficult to use a single LNB or waveguide to cover this wider frequency range, the frequency range being covered by two or more LNBs which are tuned to cover part of the frequency range, for example 10.7-11.8 GHz. and 11.7-12.2 GHz. The existing LNB may be frequency limited because of the bandwidth achieved by the reflection rotation of the existing design.
JP-A-02029001 discloses a waveguide system which is used to rotate and reflect a signal. One embodiment of this system uses a stepped dielectric plate, which is non-reflecting, to introduce a phase shift of 180.degree. for one component of the signal relative to the orthogonal component. This reference discloses an alternative embodiment which uses a capacitive metal rod or a dielectric rod on the diagonal line of the waveguide cross-section instead of a stepped dielectric plate. The particular solution to this problem may require a dielectric plate or rod or a capacitive metal rod.
GB 2 076 229 discloses the use of a stepped plate in apparatus for converting circularly polarised signals in a square waveguide into linearly polarised signals. It is a modified form of septum polariser which is well known in the art, and may not relate to reflection and recombination of signals to provide an increased frequency range of operation.
FR 2 615 038 discloses a waveguide with a vane which acts as a short circuit to one of the coaxial probes. The apparatus may not provide phase rotation and recombination and may not be suitable for providing an increased frequency range of operation.